There’s a reason Mr. Obama seems particularly interested in rediscovering his mojo with the college set. The votes in those towns may be critical to him in a what’s shaping up to be a close campaign – much more important than they were four years ago. As he hops from quad to quad, however, he may find repeating his 2008 campus magic to be tougher.

The legend of the 2008 presidential campaign goes something like this. Candidate Barack Obama rode to victory with support from across America, but especially from a hyper-engaged “youth vote,” as kids on college campuses turned out in massive numbers.

There are a few assumptions in that version of the story. First, the college vote was especially motivated in 2008. Second, the college vote moved in different ways than the electorate as a whole. And on closer examination, there’s some mythology in those assumptions.

Patchwork Nation, our demographic/geographic breakdown of America’s counties, has a special category for communities that have larger college-age populations and that also tend to vote more Democratic as a whole, Campus and Careers. Those 76 counties don’t account for all university towns, but they hold some of the best-known big state universities – Wisconsin, Michigan, Colorado, North Carolina, Virginia, Penn. State – and serve as a reasonable proxy for the college-town vote. They are in dark green on the map below.

The vote in those Campus and Career counties was indeed up in 2008, but not dramatically – only by about 1 percentage point over 2004. In 10 states that look to be key in 2012 – Colorado, Florida, Iowa, Michigan, New Hampshire, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin – the increase in the Campus counties was about the same, about one percentage point.

OK, that’s colleges. But what about the larger pool of “youth voters”? They followed much the same pattern. Voters ages 18 to 29 made up about 18% of the 2008 electorate. They made up 17% in 2004. There was no flood of younger voters. So all those measures of “enthusiasm” among younger voters you are currently reading about may miss the point.

The real reason the president is spending so much time on college campuses isn’t turnout, it’s support. It was the margin in those counties that really mattered for Mr. Obama in 2008.

Sen. John Kerry won the Campus and Career counties in those 10 swing states by about 17 percentage points and 300,000 votes in 2004. Four years later, Mr. Obama won them by 28 percentage points and more than 500,000 votes.

In 2008, that wasn’t such a big deal. Mr. Obama saw big vote increases across the age and income spectrum compared with Mr. Kerry. In fact, the increase in Mr. Obama’s vote out of those Campus counties pretty closely tracked his increase in other places – suburbs, exurbs and rural locales. College-town voters were simply following a trend.

In 2012, the Obama team wants something more difficult to deliver. They want those Campus counties to buck the trend.

Most analysts project this election will be much closer than 2008 and a few hundred thousand Campus county votes out of those 10 swing states could mean the difference between capturing three of them, or five of them, or seven of them.

Look, for example, at the map below. Those two dark blue dots in the middle of North Carolina are Orange and Durham counties – home of the University of North Carolina and the Research Triangle area – and you could argue Mr. Obama won the state because of them.

Mr. Obama won the state of North Carolina by a narrow 34,000 votes in 2008. His margin of victory in those two counties alone was more than 100,000 votes. In 2004, Mr. Kerry also won Orange and Durham, but only by about 62,000 votes.

Of course, even if Mr. Obama can draw those same big margins out of those collegiate counties it’s not clear it would compensate for votes he will likely lose in other places. But the point is without those big college-town margins, he has essentially zero chance of winning the state.

That’s the arithmetic the Obama team is figuring in North Carolina and elsewhere.

The Obama victory margin was bigger in other swing states – 146,000 in Iowa, 415,000 in Wisconsin, 621,000 in Ohio – but those margins too will likely be much tighter this year. What’s more, a lot of those states don’t have many young people. In eight of the 10 less than 25% are between the ages of 18 and 34.

For Mr. Obama, the Campus counties, with their more liberal stances on social issues and their younger voters, are places draw a line in the sand and try to hold onto his 2008 margins.

Will it work? Bucking a trend is much harder than riding one. But you can expect to see Mr. Obama’s college tour rolling on through November.

About Washington Wire

Washington Wire is one of the oldest standing features in American journalism. Since the Wire launched on Sept. 20, 1940, the Journal has offered readers an informal look at the capital. Now online, the Wire provides a succession of glimpses at what’s happening behind hot stories and warnings of what to watch for in the days ahead. The Wire is led by Reid J. Epstein, with contributions from the rest of the bureau. Washington Wire now also includes Think Tank, our home for outside analysis from policy and political thinkers.